One of the main defects which appear in intermediate-voltage electrical cables is the formation of water trees. The water tree is an interference phenomenon which appears in cables insulated with crosslinked polyethylene. This phenomenon can generally take on two forms, namely the bow tie tree or the half-bow tie tree, also called frayed or open (vented tree). The formation of a water tree requires the combined presence of an alternating electrical field and of water. It arises from an insulation defect and propagates in the insulation until it ends in its breakdown. The lifetime of a water tree is very variable. It is between a few months and a few years. Three stages in the life of a tree can be discerned: an incubation period, a growth period, and the period of change into the electrical tree which causes the breakdown. Under the microscope the structure of the water tree is seen as a group of microchannels which are more or less mutually connected.
To try and overcome these disadvantages it has first of all been recommended to use hydrophilic material in combination with the crosslinked polyethylenes, these being intended to delay the appearance and the growth of the trees. Next, compounds of the silane type or copolymers of ethylene and alkyl acrylate, such as ethyl or butyl acrylates, have been used in combination with these polyethylenes. Thus, Japanese Patent JP-A-02 311121 describes, as electrical insulation for cable, a mixture of polyethylene with an ethylene-acrylate copolymer or an ethylene-acrylic acid copolymer or an ethylene-vinyl acetate copolymer grafted with maleic acid or an ethylene-acrylate copolymer grafted with maleic acid.
In practice, however, it has been found that previously recommended solutions are not entirely satisfactory, because breakdowns of electrical cables as a result of formation of a water tree continue to occur much too frequently.